1. Field of Invention
The invention relates to a startup circuit of a power supply. More particularly, the invention relates to a startup circuit that may provide a discharging path for an electromagnetic interference (EMI) filter in a power supply.
2. Description of Related Art
FIG. 1 illustrates circuitry of a conventional power supply. The power supply 100 includes capacitors C1, C2, and CO, a transformer L1, a resistor RD, and a bridge rectifier BR1. The capacitors C1 and C2 and an inductor of the transformer L1 in the power supply 100 together constitute an inductance/capacitance (LC) electromagnetic interference filter.
According to the structure of the power supply 100, after the power supply 100 stops receiving alternating current (AC) input power ACIN, the electromagnetic interference filter in the power supply 100 still stores a fixed amount of charges. To discharge the electromagnetic interference filter, the conventional power supply 100 is equipped with the resistor RD that is connected to the capacitor C1, so as to perform the discharging action. This resistor RD, however, also consumes power while the power supply 100 normally functions. In particular, when the power supply 100 is not loaded, the resistor RD is the main power consumption source of the power supply 100.